Apparatus for Automated Opening of Craniotomies for Mammalian Brain Access

ABSTRACT

An automated craniotomy opening apparatus includes a drilling apparatus with a drilling tip, at least one drilling apparatus positioning device, a detection device, and a computer processor that automatically controls the drilling apparatus, the positioning device, and the detection device. A method for automated opening of craniotomies includes, under automatice control of a computer processor, drilling into a skull for a predetermined distance and determining when there is a conductance drop near the drilling tip that indicates skull breakthrough. If the conductance is not below a predetermined threshold, drilling continues iteratively manner until conductance is below the threshold. A craniotomy pattern may be predetermined and automatically drilled under control of the processor. A cranial window may be created by drilling along a path that interpolates between holes to form the circumference of the window. Determining conductance may include use of an impedance detection circuit.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/096,256, filed Apr. 11, 2016, now U.S. Pat. No. 10,820,914, issuedNov. 3, 2020, which claims the benefit of U.S. Provisional ApplicationSer. No. 62/146,201, filed Apr. 10, 2015, the entire disclosures ofwhich are herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant Nos.NS087724, R01 EY023173, R01 MH103910, and R43 MH101943, awarded by theNational Institutes of Health, and under Grant Nos. CBET1053233 andDMS1042134, awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED IN COMPUTER-READABLEFORMAT

This application contains a computer program listing appendix submittedin computer-readable format as an electronically-filed text file underthe provisions of 37 CFR 1.96 and herein incorporated by reference. Thecomputer program listing appendix text file includes, in ASCII format,the following files: gui.py, neurostar.py, nidaq.py, androbot_retinotopy._GUI.py.

FIELD OF THE TECHNOLOGY

The present invention relates to medical procedures and, in particular,to automated craniotomies.

BACKGROUND

Many neuroscience techniques, such as optogenetics and in vivoelectrophysiology, require access to the brain. Ideally, craniotomiescould be performed in a repeatable and automated fashion, withoutdamaging the underlying brain tissue.

Automation of craniotomies could in principle enable in vivoneuroscience experiments to be performed with greater ease,reproducibility, and throughput than it is possible for human surgicaloperators to achieve. These benefits could in turn result in betterrepeatability of experiments and higher quality neural data, as well asthe ability to deploy neural recording or stimulation probes in complex3-D geometries that target multiple brain regions [Zorzos, A. N.,Scholvin, J., Boyden, E. S., & Fonstad, C. G. Three-dimensionalmultiwaveguide probe array for light delivery to distributed braincircuits. Opt Lett. 37(23): 4841-4843, 2012]. In vivo whole cell patchclamp neural recording [Kodandaramaiah S. B., Franzesi G. T., Chow B.Y., Boyden E. S., Forest C. R. Automated whole-cell patch-clampelectrophysiology of neurons in vivo. Nat Methods. 9(6): 585-587, 2012]has previously been automated, discovering that a glass micropipettebeing lowered into the living mouse brain undergoes a stereotyped smallincrease in pipette resistance upon encountering a cell, enablingbuilding a robot that can automatically patch clamp neurons in theliving mammalian brain.

Automated craniotomies have been attempted before. Some current methodsfor performing automated craniotomies use force feedback [Loschak, P.,Xiao, K., Pei, H., Kesner, S. B., Thomas, A. J., and Walsh, C. Cranialdrilling tool with retracting drill bit upon skull penetration,Proceedings of the Design of Medical Devices Conference, Apr. 10-12,2012, Minneapolis, Minn.: University of Minnesota Medical DevicesCenter, 2012; Pohl, B. M., Schumacher, A., & Hofmann, U. G. (2011).Towards an automated, minimal invasive, precision craniotomy on smallanimals. 2011 5th International IEEE/EMBS Conference on NeuralEngineering, 302-305], imaging to map out the skull geometry and thenopen loop operation [Cunha-Cruz, V., Follmann, a, Popovic, a, Bast, P.,Wu, T., Heger, S., Radermacher, K. (2010). Robot- and computer-assistedcraniotomy (CRANIO): from active systems to synergistic man/machineinteraction. Proceedings of the Institution of Mechanical Engineers,Part H: Journal of Engineering in Medicine, 224(3), 441-452], andfemtosecond lasers to ablate the skull [Jeong, D. C., Tsai, P. S., &Kleinfeld, D. (2013). All-optical osteotomy to create windows fortranscranial imaging in mice. Optics Express, 21(20), 23160-8]. However,it is unclear whether these methods can achieve better than millimetricresolution. Open-loop systems require CT scanning of the skull tomeasure the skull thickness, which is both expensive and involvesdangerous x-rays. Use of femtosecond lasers is also an expensiveproposition.

SUMMARY

When drilling through a skull, a stereotypical increase in conductancecan be observed when the drill bit passes through the skull base, with asudden increase in the electrical conductance between the drill and bodyindicating when the drill is in the skull but not touching the brain.Lowering a drill through the skull until an increase in the conductancebetween the drill and the body indicates that the drill is in the skullis of use in automating craniotomy surgeries. The present inventionemploys a methodology that leverages this discovery.

A robot was implemented to perform automated craniotomies usingconductance measurements, and the precision of the drilling wascharacterized. Using the invention, craniotomies can be done reliably,and without any bleeding from the meninges or brain, even in preciselyaligned arrays. A commercially available motorized stereotaxic apparatuswas modified and outfitted with a measurement circuit based on the sameprinciple. Using the same detection method, it is possible to createlarger windows in the skull by drilling multiple small craniotomies in aring, interpolating in between them via milling, and removing the skullpiece thus isolated. Robots utilizing this approach may find widespreaduse for in vivo neuroscience experiments that require either largecranial windows or multisite injector, electrode, fiber, or other deviceinsertion through arrays of craniotomies.

In one aspect, the invention is an architecture for a robotic devicethat can perform this methodology, along with two implementations—onebased on custom hardware, one based on commercially availablehardware—that can automatically detect such changes, and create largenumbers of precise craniotomies, even in a single skull. This techniquecan be adapted to automatically drill cranial windows severalmillimeters in diameter. Such robots are not only useful for helpingneuroscientists perform routine craniotomies more reliably, but can alsobe used to create precisely aligned arrays of craniotomies that would bedifficult or impossible to drill by hand.

In one aspect of the invention, a method for automated opening ofcraniotomies includes the steps of positioning a craniotomy apparatusdrilling tip at a starting position relative to a target skull andperforming a series of steps under the control of a computer processorconfigured with control software for operating the craniotomy apparatus.The steps include drilling into the target skull with the drilling tipfor a predetermined distance; after drilling, determining theconductance near the drilling tip; if the conductance is below apredetermined threshold, returning the drilling tip to a home position;and if the conductance is not below the predetermined threshold,repeating the steps of drilling and determining until the conductanceexceeds the predetermined threshold. The step of determining theconductance may include, under the control of the computer processor,measuring impedance with an impedance detection circuit and calculatingthe conductance using the measured impedance. The step of measuringimpedance may include sending a signal through the impedance circuit tothe drilling tip; detecting a voltage at the target skull; and sending asignal representing the detected voltage from the impedance circuit tothe computer processor, or may include sending a signal at apredetermined voltage through the impedance circuit to the target skull;detecting a voltage at the drilling tip; and sending a signalrepresenting the detected voltage from the impedance circuit to thecomputer processor. The step of calculating may include determining avoltage drop across the impedance circuit.

The method may include predetermining a drill hole pattern comprising aplurality of craniotomies to be drilled in the target skull and creatingthe drill hole pattern by drilling the plurality of craniotomies. Thepredetermined drill hole pattern may be selected to facilitate creationof a cranial window in the target skull. The cranial window may becreated by drilling along a path that interpolates between the holes toform the circumference of the cranial window.

In another aspect of the invention, an automated craniotomy openingapparatus includes a craniotomy drilling apparatus with a drilling tip,at least one craniotomy drilling apparatus positioning device connectedto the craniotomy drilling apparatus, a detection device connected tothe drilling tip or drill base, and a computer processor speciallyconfigured and connected to control the craniotomy drilling apparatus,the craniotomy drilling apparatus positioning device, and the detectiondevice. The computer processor is programmed to perform the steps ofpositioning the drilling tip with respect to the target skull at apredetermined position for drilling; drilling into the target skull withthe drilling tip for a predetermined distance; after drilling,determining the conductance near the drilling tip using the detectiondevice; if the conductance is below a predetermined threshold, returningthe drilling tip to a home position; and if the conductance is not belowthe predetermined threshold, repeating the steps of drilling anddetermining until the conductance exceeds the predetermined threshold.The detection device may include an impedance detection circuit, whichmay include a signal source, a sense resistor in series with the signalsource, and an output for sending the detected impedance to the computerprocessor. The detection device may be attached to the drilling tip in amanner that permits measurements to be made while the drilling apparatusis running. The drilling tip may be a blunt-tipped end mill. Thepositioning apparatus may be a motorized stereotaxic stage connected tothe drilling apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, advantages and novel features of the invention willbecome more apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,wherein:

FIG. 1 is a flowchart of an embodiment of a method for craniotomyautodrilling, according to one aspect of the invention.

FIGS. 2-6A-B depict aspects of the design and implementation of anexample autodrilling robot for performing the method of FIG. 1,according to one aspect of the invention, wherein:

FIG. 2 is a diagram of an example electrical impedance measurementcircuit suitable for use in the invention,

FIGS. 3A-B illustrate an example experimental setup suitable for use inperforming the method,

FIG. 4 depicts an example custom-built autodrilling robot according toone aspect of the invention,

FIG. 5 depicts example drill bits usable in the implementation of FIG.4,

FIG. 6A is a graph of normalized electric potential across the drill andmouse, as a function of frequency, as a dental burr in an exampleexperimental setup is lowered into the skull for seven different mice,and

FIG. 6B is a graph of electrical conductance vs. frequency for theexperiment of FIG. 6A.

FIGS. 7-10A-B depict implementation aspects of the method forautodrilling, and validation thereof, when run on the robot of FIGS.2-5, according to further aspects of the invention, wherein:

FIG. 7 is a representative CT scan of a skull, exhibiting anexperimental drilling pattern used to produce the data of FIGS. 8A-B,

FIG. 8A is a plot of craniotomy hole size as a function of finalstopping normalized electric potential using the drilling pattern ofFIG. 7 and the custom-made automated craniotomy robot,

FIG. 8B is a plot of craniotomy hole size vs. electrical conductance,for the data of FIG. 8A,

FIG. 9 is a representative CT scan of another skull, exhibiting anexperimental drilling pattern used to produce the data of FIGS. 10A-B,

FIG. 10A is a plot of craniotomy hole size as a function of finalstopping normalized electric potential using the drilling pattern ofFIG. 9 and the custom-made automated craniotomy robot, and

FIG. 10B is a plot of electrical conductance vs. craniotomy hole size,for the data of FIG. 10A.

FIGS. 11-20 relate to a specific example implementation of acustom-built system according to one aspect of the invention, wherein:

FIG. 11 depicts minigrabbers that are used to connect to the drill andto the body of the animal in the example implementation of a deviceaccording to the invention,

FIG. 12 depicts the front panel of the control display used in theexample implementation,

FIG. 13 is a schematic flowchart depicting the steps by which theexample system initializes the motors and then waits for the user toposition the drill in the correct location,

FIGS. 14A and 14B are schematic flowcharts depicting the steps by whichthe example system moves the drill into position and prepares to drill,

FIG. 15 depicts the pattern in which the drill is moved in a preferredembodiment,

FIG. 16 is a schematic flowchart depicting the steps by which theexample system implements an optional procedure intended to savedrilling time,

FIGS. 17A and 17B are schematic flowcharts depicting the steps by whichthe example system performs the automated drilling,

FIGS. 18A and 18B are schematic flowcharts depicting the steps by whichthe example system determines whether or not drilling should beperformed, and

FIGS. 19 and 20 are schematic flowcharts depicting the steps by whichthe example system retracts the drill and completes the program.

FIGS. 21-24 relate to a specific example modified commercial systemimplementation according to one aspect of the invention, wherein:

FIG. 21 is a schematic of an example implementation of a system forperforming automated craniotomies utilizing a modification of theNeuroStar motorized stereotaxic, according to one aspect of theinvention,

FIGS. 22A-B are side and top views that depict steps for creating anexample implementation of the impedance measurement device suitable foruse in various implementations of the invention,

FIG. 23 is an example implementation of a measurement circuit useable inthe modified commercial system implementation, and

FIG. 24 is a screenshot of an example embodiment of a graphical userinterface useable with the modified commercial system implementation.

FIGS. 25A-B and 26A-D depict an exemplary procedure for drilling largecranial windows according to other aspects of the invention, wherein:

FIGS. 25A and 25B depict windows that were created after drilling aseries of test holes, and

FIG. 26A-D depict the steps involved in creating two cranial windows inan actual mouse skull.

FIGS. 27A-B and 28A-B depict example results from autodrillingexperiments according to aspects of the invention, wherein:

FIG. 27A is a graph of normalized electric potential vs. distancetraveled, for 10 holes in one mouse skull,

FIG. 27B is a graph of electrical conductance vs. distance traveled forall 10 craniotomies of Sig. 27A,

FIG. 28A is a plot of hole size, measured at the base of the skull,measured using x-ray micro-computed tomography (CT), as a function offinal normalized electric potential, with the drill stopping whenvarious normalized electrical potentials were reached, and

FIG. 28B is a plot of craniotomy hole size vs. electrical conductancefor the data of FIG. 28A.

DETAILED DESCRIPTION

The invention is method for performing precise craniotomies, based onconductance measurements taken while lowering a drill bit through theskull, along with devices for performing the method. When drillingthrough the skull, a stereotypical increase in conductance can beobserved when the drill bit passes through the skull base. A circuit isused to measure this change in order to precisely determine when to stopdrilling. This method has been applied to robotic drilling devices thatcan automatically stop at the precise moment when the skull has beendrilled through. This prevents damage to the brain and allowsresearchers to create small, closely spaced craniotomies with littletraining. Also, this method has been used to create complexthree-dimensional craniotomies by recording the depth location ofseveral small craniotomies in order to create a profile of the innersurface of the skull. The method can further be advantageously adaptedto automatically drill cranial windows several millimeters in diameter.

In one aspect, the invention is an architecture for a robotic devicethat can perform the method of the invention. Two specific embodimentsof the device have been implemented—one based on custom hardware and onebased on commercially available hardware. The device can automaticallydetect conductance changes and create large numbers of precisecraniotomies, even in a single skull. Such robots are not only usefulfor helping neuroscientists perform routine craniotomies more reliably,but can also be used to create precisely aligned arrays of craniotomiesthat would be difficult or impossible to drill by hand.

The automated craniotomy robot uses a signal to automatically detectwhen the drill bit has gone through the skull so that it can stopdrilling and not damage the brain. A small AC voltage (1 mV) is sentthrough the drill bit and the body of the mouse and the amplitude ofthis signal is measured by a computer program. Bone is a very goodelectrical resistor, so that when the drill bit pierces through theskull, a noticeable increase in conductance signifies when to stop therobot from drilling further down.

FIG. 1 is a flowchart of an embodiment of the method for autodrilling,according to one aspect of the invention. As shown in FIG. 1, the drillis moved 105 to the starting position, i is set 110 to 1, and the drillis moved 115 to location x(i), y(i), z(x(i), y(i)). V_(n) is measuredand V_(n)/V_(n) max calculated 120 and compared 125 to the threshold. Ifthe result is not below 130 the threshold, the apparatus drills down 135a prespecified amount and then returns to measurement and calculationstep 120. If the result is below 140 the threshold, the drill isretracted 145 and i is incremented and compared 150 to a maximum i (N).If not greater 155 than the maximum, the drill is moved 115 relative tothe new value of i. If i is greater 160 than the maximum, the drill isreturned 165 to the home position.

An impedance detection circuit, threshold value, and techniques forminimizing the current used are all aspects of physical implementationsof the invention. Preferred embodiments of the impedance detectioncircuit utilize components that can be found in most laboratories andcan be added on to an existing surgery setup with only a fewmodifications.

The threshold value may be determined through trials and should bechosen to be a balance between being high enough to ensure that thedrill does not go too far and damage the brain, but low enough toconsistently create craniotomies without residual bone being left behindthat could damage probes or impede visual observations of the brain.

FIG. 2 is a diagram of an example electrical impedance measurementcircuit suitable for use in the invention. The basic impedance circuitconsists of a sense resistor, an AC signal source, and a computerprogram to capture and detect the signal. Shown in FIG. 2 are senseresistor 210, function generator (or DAQ) 220, parasitics 230, mouseresistance 240, and DAQ analog input 250. It will be clear to one ofskill in the art that, while specific components are set forth in thisexample, other components providing the same functionality would besuitable and could be advantageously employed in the construction of animpedance circuit usable in the invention.

Several techniques have been implemented in order to minimize thecurrent necessary for the impedance detection circuit to work. Given the200 μm diameter drill bits typically used, the current density levelsare nearly two orders of magnitude below the lowest current densityvalues published capable of stimulating brain tissue. The sense resistorwas specifically chosen to be large enough to minimize the amount ofcurrent applied (so as to ensure no damage to the brain or stimulationof neurons), but still allows the detection of the signal. A Fouriertransform is used to detect the signal that might otherwise be too smallto measure.

In an example implementation, the circuit works by sending a sine wavefrom either a function generator (such as, but not limited to, part33250A, Agilent, Santa Clara, Calif.) or from a LabVIEW (LabVIEW 2011,National Instruments, Austin, Tex.) data acquisition board (DAQ) (NIUSB-6353, National Instruments, Austin, Tex.), or other similar device,through a sense resistor (681 kW for FIGS. 6A-B) and a wire in contactwith either the drill body or directly in contact with the drill bit.The cable conducts the electrical signal from the function generator orDAQ to the drill and from the mouse back to the oscilloscope or the DAQ.This cable consists of a coaxial cable, a pair of twisted wires, or ashielded USB cable. The wire mesh shields of the coaxial and USB cableswere connected to earth ground to minimize the effects of environmentalnoise. The rear paw of the mouse made electrical contact to the groundlead of this cable connected to an oscilloscope (TDS 2024C, Tektronix,Beaverton, Oreg.) or the ground pin of the DAQ, through a piece of metal(contact area of 5 mm²) touching the skin of the paw, and heldstationary by a test clip whose spring had been stretched to make itweaker.

Two prototype systems have been implemented, tested, and verified: acustom-built system (FIGS. 3A-B-20) and a system (FIGS. 21-24) createdby adapting and extending a commercially available device. Craniotomiesare now automatically being made in the skulls of mice on a routinebasis.

Custom-Built Embodiment (FIGS. 3A-B-20)

A custom-built system was constructed that consists of an air-powereddental drill (PR-304, NSK, Tokyo, Japan) mounted on a three-axiscomputer-controlled stage equipped with three motors (PT3/M-Z8 stage,TDC001 controllers, TCH002 power supply, Thorlabs, Newton, N.J.), usinga 3D-printed, electrically-insulating mount made of acrylonitrilebutadiene styrene (ABS), with an electric potential detection circuitrunning through the drill bit. While an air-powered drill is describedin conjunction with this embodiment, it will be clear to one of skill inthe art that an electric drill, such as, but not limited to, anelectrical micromotor carving tool is also suitable for use in theinvention.

FIGS. 3A-B together are an illustration of an experimental setupsuitable for use in performing the method. Shown in FIG. 3 are wire 305attached to grounded paw 310 of mouse 315, wire 320 attached to drillbit 325 of drill 330 having air power input 335, electrical contact tobit 340, mouse skin 345, skull 350, fluid and meninges 355, and mousebrain 360.

While in the depicted embodiment the AC signal is sent through the drillbit and the ground is on the paw of the animal, it is also possible tosend the signal through the paw and have the drill bit be the ground.This alternate arrangement provides a common ground with the drill sothat the electrical noise can be lowered.

Although optional, it has been found that the best results are obtainedby using head-fixed animals, with a metal plate being attached to theskull of the animal and held securely so that there is minimaldetectable motion of the skull with respect to the drill.

FIG. 4 depicts a custom-made autodrilling robot implemented according toone aspect of the invention. Shown in FIG. 4 are electrically insulatingdrill mount 410, horizontal motorized linear stage 420, verticalmotorized linear stage 430, compressed air input 440 for air powereddental drill 450, and electrical contact 460 to drill bit 470.

The motors used with the motorized three-axis stage have a repeatablestep size of 200 nm, a travel distance of 25 mm, and are controlled bythe DAQ, driven with LabVIEW commands. Usually, step sizes of 5 μm inthe z-direction were used, aiming for a resolution comparable to that ofa cell diameter, so the motor precision used was probably more than whatis needed. Thus, implementations that are several-fold cheaper thanpresented here might well be assembled; the motors in the exampleimplementation were chosen to allow exploration of the parameters ofautodrilling. A common lab air supply was used to power the dentaldrill, and a solenoid valve (EV-2-6, Clippard, Cincinnati, Ohio) wasconnected between the air supply and the dental drill so that the dentaldrill could be turned on and off through a digital signal from the sameLab VIEW program.

FIG. 5 depicts various drill bits found to be usable in theimplementation of FIG. 4, including a commercially available dental burr510, 500 μm in diameter, a custom drill bit 520 fusing a 200 μm tip witha custom aluminum dental drill adapter, and a custom 200 μm end mill 530created by using a lathe to turn down a commercially available bit.Scale bar, 1/16″. While specific drilling bits are pictured, it will beclear to one of skill in the art of the invention that suitable drillbits useable in the invention are not limited to the ones depicted inFIG. 5.

Dental drills are inexpensive and capable of extremely high rotary rateswith minimal vibration, which is why they are popular in neurosciencefor making small craniotomies. Most dental drills have a standardopening of 1/16″ that fits commercially available dental burrs. Theseburrs range variously in size and shape, but are only available down to500 μm in diameter. Commercially available drill bits come in sizes of200 μm diameter and less (McMaster-Carr), but have a 1 mm diameter shankthat is too small to fit into the dental drill. It was desirable tocreate very small craniotomies, so a custom adapter was built thatbridged this 1 mm to 1/16″ gap in order to use these drill bits. Toproduce these adapters, a lathe was used to first drill a 1 mm hole in a¼ in diameter aluminum rod. Next, this rod was turned down with thelathe to an outer diameter of 1/16″ and cut to 25 mm in length. Thedrill bits were then cut down to about 10 mm in length using a grindingtool, and the drill bit was then press fit into the adapter. One ideaexplored was a custom-made chuck that would allow various diameter drillbits to be used with the dental drill. This idea was abandoned becauseit was found that the high speed of the dental drill (up to a nominal320,000 rpm) requires a precisely balanced chuck to eliminatevibrations; the adapters produced in the example implementation are, incontrast, quite inexpensive (a few cents of material cost per adapter)and quick to produce (˜15 minutes each). However, it is clear that useand/or creation of a chuck or any of the many other types of adaptorsknown in the art is within the skill of the artisan and therefore withinthe scope of the invention.

The drill bits used in the example implementation have a diameter of 200μm and a point angle of 118°. This angled cutting edge means that, for afully bored hole in the skull, the drill bit must penetrate 60 μm beyondthe inner surface of the skull, increasing the risk of damage. This isan issue with all pointed drill bits used in neuroscience, not justthose being used with dental drills.

Miniature square end mills (Harvey Tools) could result in moreconsistent craniotomy openings, but are not typically used inneuroscience applications. End mills with a 200 μm diameter and ⅛″ shankwere turned down by a machine shop in order to be able to fit into the1/16″ dental drill opening. Since these have a flat end, a fully boredout craniotomy is created when the circuit detects breakthrough of theskull. For the same reason, these are potentially less damaging to thebrain as well: since they are not pointed, they do not need to extendbeyond the base of the skull to complete full craniotomy. End mills alsoallow for cutting in all three directions so more elaborate craniotomiescan be created.

Calibrating the stereotaxic coordinates can be performed by moving thedrill tip to bregma or lambda for a well-aligned mouse in a stereotaxicframe, and then zeroing the coordinates in the software. Use of ahead-fixed mouse was found to produce the best results.

In the original custom-built system, accurate measurements ofdrill-to-body potential could not be made while the drill bit wasspinning, presumably due to poor electrical contact between a spinningbit and its surroundings. Therefore, the drill was always turned off,waiting for the drill to stop spinning, before taking a measurement ofthe drill-to-body electrical potential. This problem was solved in asubsequent implementation, which employs a ball bearing installed on thedrill bit to which the signal wire is attached, allowing continuousimpedance testing without the need to stop the drill. The inside of thebearing is filled with conductive grease, in order to create electricalcontinuity between the drill bit and the signal wire.

FIG. 6A is a graph of normalized electric potential across the drill (asin FIG. 3, but with the electrical contact on the bit moved to touch thedrill itself) and mouse, as a function of frequency, as a 500 μm dentalburr is lowered into the skull for 7 different mice (step size, 10 μmfor six mice, 50 μm for the seventh. FIG. 6B is a graph of electricalconductance vs. frequency (as in FIG. 6A) for all 7 mice of FIG. 6A. Thedata in FIGS. 6A-B was obtained using a function generator (33250A,Agilent, at 20V, varying frequencies) through a sense resistor (681 kW)and a wire in contact with the drill body. A coaxial cable conducted theelectrical signal from the function generator to the drill and from themouse back to the oscilloscope (TDS 2024C, Tektronix, Beaverton, Oreg.).The oscilloscope was used to measure the voltage drop, V_(n), across themouse. This method generally only works when the signal amplitude ismuch higher than the noise, which is the case when there is a largedriving current.

Measurements of the voltage drop with a ground electrode consisting of askull screw (self-tapping screw, size 000 thread, #303 stainless steel,3/32″ length, J. J. Morris Company, Southbridge, Mass.) inserted into amanually drilled craniotomy showed no improvement in the ability todetect a craniotomy opening by the robot drilling at a second site,presumably because the conductance through the body and through thebrain are both high compared to the conductance of the skull.

The voltage drop across the sense resistor (see FIG. 2) isV_(s)=V_(in)−V_(n), and the current flow through the sense resistor isi_(s)=V_(s)/R_(s). If parasitic currents, defined as i_(p)=V_(n)/Z_(p),are present, e.g., via capacitive coupling of signal wires with groundedshielding in a cable, then some of the sense current flows through aparasitic impedance Z_(p), calculated as the ratio of V_(p)/i_(s)(measured when the mouse is not there). The current flow through themouse, when present, is i_(m)=i_(s)−i_(p). From these equations, theratio of the voltage drop across the mouse to that of the input voltage,V_(n)/V_(in), is calculated asV_(n)/V_(in)=1−R_(s)(Z_(p)+Z_(m))/(R_(s)(Z_(p)+Z_(m))+Z_(p)Z_(m)). Whenthe drill tip makes a hole in the skull, it was found that Z_(m)decreases by five orders of magnitude, and this ratio decreases by abouttwo orders of magnitude given a sense resistance of 10 MΩ.

To facilitate comparison across the multiple experimental setupsexplored, e.g., different cables and different input voltages, thevoltage was normalized across the mouse, V_(n), by the maximum recordedvoltage, V_(n max), recorded in the open loop configuration with nomouse in the circuit, before drilling began. The maximum recordedvoltage is equal to the input voltage, when twisted pairs are used, orless for the case of parasitic currents, which occur with the coaxialand USB cables.

For FIGS. 6A-B, the impedance of the drill was included in the circuitbecause the simplest method for sending a signal through the drill is toconnect a wire to the body of the drill, which is conductive to thedrill bit. This impedance would show up as a resistor in series with themouse (as in FIG. 2). Electric potential measurements were made indiscrete steps. For FIGS. 6A-B, the drill was turned, a step of 10 μm or50 μm was taken, the drill was turned off and a measurement was madeafter the drill came to a complete stop.

FIGS. 7-10A-B depict implementation aspects of the method forautodrilling, and validation of thereof, when run on the robot of FIGS.2-5.

Electrical Current. For the experiments of FIGS. 7-10A-B, the 1 mVamplitude and 10 MΩ sense resistor were chosen so that the maximumpossible electrical current through the mouse (equal to the amplitude ofthe injected sine wave divided by the sense resistance) was small (˜100pA), in order to avoid brain stimulation. Using the cross-sectional areaof a 200 μm diameter cylinder as the electrode area of the drill bit,the current density was approximately 0.0032 A/m², nearly two orders ofmagnitude less than the lowest current densities (0.28 A/m²) capable ofstimulating brain tissue [Brunoni A. R., Nitsche M. A., Bolognini N.,Bikson M., Wagner T., Merabet L., Edwards D. J., Valero-Cabre A.,Rotenberg A., Pascual-Leone A., Ferrucci R., Priori A., Boggio P. S.,Fregni F. Clinical research with transcranial direct current stimulation(tDCS): challenges and future directions. Brain Stimul. 5(3): 175-195,2012; Nitsche M. A., Liebetanz D., Lang N., Antal A., Tergau F., Paulus.Safety criteria for transcranial direct current stimulation (tDCS) inhumans. Clin Neurophysiol. 114(11): 2220-2222, 2003; Chaieb L., AntalA., Paulus W. Transcranial alternating current stimulation in the lowkHz range increases motor cortex excitability. Restor Neurol Neurosci.29(3): 167-175, 2011]. Furthermore, this current was only applied acrossthe brain for a few seconds during the drilling operation. Additionally,the signal processing methods used above could in principle be used tolower the current down further, if desired.

The circuit used to produce the data of FIGS. 8A-B and 10A-B works bysending a sine wave from a LabVIEW (LabVIEW 2011, National Instruments,Austin, Tex.) data acquisition board (DAQ) (NI USB-6353, NationalInstruments, Austin, Tex.) (1 mV, 100 Hz), through a sense resistor (10MW) and a wire directly in contact with the drill bit. The cableconducts the electrical signal from the DAQ to the drill and from themouse back to the DAQ. This cable consisted of a shielded USB cable. Thewire mesh shields of the USB cable was connected to earth ground tominimize the effects of environmental noise. The rear paw of the mousemade electrical contact to the ground lead of this cable connected tothe ground pin of the DAQ, through a piece of metal (contact area of 5mm²) touching the skin of the paw, and held stationary by a test clipwhose spring had been stretched to make it weaker.

The DAQ measures V_(n) in a different manner than for the priorembodiment. The LabVIEW program performs a partial discrete Fouriertransform on V_(n), calculating the amplitude of V_(n) assqrt(Re(X)²+Im(X)²)/N, where N=15,000 is the number of samples read, andX is the coefficient of the sinusoidal component of the measured samplesat the test frequency (100 Hz). X is defined as X=Σ_(n=0)^(N-1)x_(n)e^(−i2πkn/N) where x_(n) is the n^(th) measurement, and k iscalculated as N/(sampling rate (25 KHz) divided by the input frequency),which equates to the number of cycles that are recorded (in this case60). Re(X) is calculated as Σ_(n=1) ^(N-1)x_(n) cos(−2πkn/N), and Im(X)is calculated as Σ_(n=1) ^(N-1)x_(n) sin(−2πkn/N). This method ofmeasuring V_(n) was chosen because the driving current of about 100 pAis deliberately very small, in order to avoid any biological effects,which also makes it smaller than the noise (when examined by eye).

The impedance of the drill was removed from the circuit by having a wiretouch the shank of the drill bit directly, taking care not to applyexcessive radial forces to the bit which could cause premature wear ofthe bearings. Electric potential measurements were made in discretesteps. The solenoid valve was turned on for 300 μs, a step of 5 μm wastaken, a pause of 3.5 s allowed the drill to come to a complete stop,and then a measurement was taken for 0.6 s.

Once a threshold was defined that balanced craniotomy success andsafety, the method of FIG. 1 was implemented in order to performelectric potential measurements over time while a drill was loweredthrough the skull in 5 μm steps, halting motion when the drill-to-bodypotential dropped below the threshold. Since each drill step andmeasurement step took about 4 seconds, each craniotomy took about fourminutes.

Across multiple trials of the custom craniotomy robot and method, it wasfound that craniotomies could be reliably drilled using the normalizedelectric potential threshold of 0.65 (FIGS. 7, 9; n=72 craniotomies in 3mice). For 6 of the craniotomies, bleeding was observed from the skullafter drilling to only a shallow depth (implying again that a bloodvessel in the skull had been hit). For 4 of these cases, waiting ˜10-20minutes for the vessel to clot was sufficient to allow the procedure tocontinue to the point of a complete craniotomy.

FIG. 7 is a representative CT scan of a skull, showing the bregma 710and exhibiting the experimental drilling pattern used to produce thedata of FIGS. 8A-B. Scale bar is 1 mm. FIG. 8A is a plot of craniotomyhole size as a function of final stopping normalized electric potentialfor 72 craniotomies in 3 mice with a 200 μm drill bit, using thedrilling pattern of FIG. 7 and the custom-made automated craniotomyrobot with a step size of 5 μm and normalized electrical potentialthreshold of 0.65. Each mouse is represented by a different shape. For 2of the 72 craniotomies, the drill bit did not pass the bottom of theskull, and thus are at the y=0 line. FIG. 8B is a plot of craniotomyhole size vs. electrical conductance for the data of FIG. 8A.

FIG. 9 is a representative CT scan of another skull, showing the lambda910 and exhibiting an experimental drilling pattern used to produce thedata of FIGS. 10A-B. Scale bar is 1 mm. FIG. 10A is a plot of hole sizeas a function of final stopping normalized electric potential for 20craniotomies in 5 mice with a 200 μm flat-end end mill, using thedrilling pattern of FIG. 9 and the custom-made automated craniotomyrobot with a step size of 5 μm and normalized electrical potentialthreshold of 0.45. Each mouse is represented by a different shape. For 3of the 20 craniotomies, the drill bit did not pass the bottom of theskull, and thus are at the y=0 line. FIG. 10B (3G) is a plot ofelectrical conductance vs. hole size for the data of FIG. 10A.

The holes drilled in FIGS. 7 and 9 were typically less than the width ofthe drill bit, because the pointed tip would break through before thewider shaft, resulting in a not-completely bored out hole. As notedearlier, the use of a pointed or rounded drill bit, although popular inneuroscience, may also result in part of the drill projectingsignificantly below the skull base, potentially injuring the brain.Blunt-tipped end mills of 200 mm diameter were created that could beused in a dental drill, and the threshold used was changed to a lowervalue, 0.45. It was found that cylindrical holes could now be made, withdiameters equal to or larger than the end mill diameter (n=20craniotomies in 5 mice). For 3 of the 20 craniotomies, skull bleedingwas observed; in principle these could have been allowed to continue,after a waiting period to allow clotting. Craniotomy sizes of greaterthan 200 μm were due to some end mills not being perfectlyconcentrically machined down, in the machining process used to modifythe end mills.

For the end mill experiments, the precision of the craniotomy robot wascharacterized. Four craniotomies in an array were created, with 500 μmspacing (center-to-center) in both the anteroposterior and mediolateraldirections. The measured center-to-center distance was 496±6 μm(mean±std. dev., n=10 pairs of craniotomies, including the partialcraniotomies) in the mediolateral direction, and 492±10 μm in theanteroposterior direction. These errors approach the resolution of theCT scanner used to image the skulls (˜5 μm).

The two steps in this procedure that required manual intervention werethe initial alignment of the drill bit with the center of the desiredwindow, and the removal of the circular bone fragment under saline. Inbetween these steps, the robot operated independently. In someinstances, the drill passed through a skull vessel, causing bleedingthat quickly subsided. In no cases was damage to the vessels of themeninges or the brain itself observed.

Design, construction, and operation of a “custom-built” automatedcraniotomy robot.

In one particular implementation, an automated craniotomy robot consistsof a voltage detection circuit, a drill, and actuators. Table 1 is anexample parts list for an embodiment of a custom-built implementation ofthe present invention.

TABLE 1 Description Vendor Model # air powered dental drill NSK PR-304DAQ National Instruments NI USB 6353 solenoid valves Clippard EV-2-6 xyzstage + motors Thor Labs PT3/M-Z8 motor controller Thor Labs TDC001motor controller power Thor Labs TCH002 supply 200 μm drill bit McMaster8904A27 200 μm end mill Harvey Tool 13908 drill holder customstereotaxic Kopf 900 10 MΩ resistor various shielded USB cable various

The data acquisition board (DAQ board Model: NI USB-6353) is used tointeract with LabVIEW software. Specifically, an analog output is usedto generate the AC signal for the detection circuit. An analog input isused to measure this signal, and a digital output is used to turn thedrill on and off with a solenoid valve.

The sense resistor (10 MΩ) is connected directly on the DAQ board tominimize noise. The sense resistor is large so that the maximum currentremains low.

A shielded USB cable is used to send the AC signal to and from themouse. Without a shielded cable, environmental noise would dominate thesignal since a very small amount of current HO pA) is used in thedetection circuit. The ground of the USB cable is connected to earthground.

As shown in FIG. 11, minigrabbers are used to connect to the drill andto the body of the animal. These have been modified to decrease theirapplied force by stretching their springs. The unshielded wire distanceis minimized for noise purposes.

In a preferred embodiment, an air-powered dental drill (NSK Presto) thathas the drill axis aligned with the body of the dental drill isemployed. Unlike most dental drills that have a 90° bend between thehandle and the drill axis, this drill does not get in the way of thestereotaxic frame. It will be clear to one of skill in the art thatthere are other dental drills that would be suitable for use in theinvention, as well as other types of drills. The NSK Presto dental drillwas selected because it has almost no run-out, unlike the first dentaldrill that was tried (Buffalo No. 220).

The electrically insulated dental drill holder is a custom 3D printedpart that secures the dental drill to the three-axis stage. It is madeof plastic (ABS), which prevents the detection signal from travellingthrough the stage. This part was initially designed for the first dentaldrill tried, but can also secure the NSK Presto. It is clear that it iswithin the skill of the artisan to create a custom holder for whateverdrill is selected for use in the invention.

A three-axis stage with linear servomotors (Model: PT3-Z8) has 1″ oftravel in each direction. These motors have a repeatable step size of0.2 μm. The program steps down in 5 μm steps so this level of resolutionis most likely unnecessary. These motors are also controllable throughLabVIEW.

Power supply for motors (e.g. TPS001, TPS008, TCH002). Each motorcontroller needs to receive power and send/receive signals from thecomputer. The cheapest option found was an individual power supply foreach motor controller. In this case, each motor controller will alsohave to be connected to the computer through a USB cable. The mediumcost option is a single power supply for up to eight motor controllers.In this option, each motor controller still needs to be connected to thecomputer through a USB cable. Another suitable alternate option is apower supply and base for up to six motor controllers. The base connectsall of the motor controllers to the computer through a single USB cable.While specific motor configurations are described, any of the manymotors and motor configurations known in the art would be suitable foruse in the invention.

A stereotaxic (Kopf Model 900) is used to secure the animal duringsurgery, as in a normal surgery setup. Unlike some other models, theKopf Model 900 has a lot of space for the drill to fit, but othersterotaxics, and other methods of securing the animal, would also besuitable.

A solenoid valve (such as, but not limited to, Clippard EV-2-6) is usedto turn the dental drill on and off through computer commands.

Since the DAQ typically cannot provide enough current to activate thesolenoid valve, a transistor and DC power supply may also be requiredwhen using a DAQ.

Commercially available dental burrs come in various shapes and sizes butonly go down to 500 μm in diameter. Miniature drill bits and end mills(200 μm drill bit from McMaster 8904A27; 200 μm flat end mill fromHarvey Tool 13908) are used to create craniotomies of 200 μm diameter(both the drill bits and end mills come in various sizes, starting at 50μm in diameter). Since dental drills currently come with a standardchuck size that will only accept 1/16″ diameter shanks, and these drillbits have 1 mm shanks, custom aluminum sleeves with a 1 mm ID and a1/16″ OD are machined. The end mills come with a ⅛″ shank. These bitsare ground down. This should be done with special tooling since the bitsare made of tungsten carbide, and it is very important that theconcentricity of the bit shank and cutting surfaces is maintained whenthe shank diameter is reduced. Also, a wire is required to make directcontact with the drill bit for the detection circuit.

Other parts used for the prototype implementation include, but are notlimited to, a computer with LabVIEW software, wall air supply, standardsurgery equipment, a secure mount for 3-axis stage, and stereomicroscope

LabVIEW Program Description/Walkthrough. Before running the program, theuser inputs how many craniotomies are to be drilled by entering thenumber of rows and columns. Also, the spacing between craniotomies inthe rows and columns is entered. Finally, a file path for a data log isentered if it is desired. The data log creates a text file for eachcraniotomy that indicates the voltage amplitude at each drill depth.

FIG. 12 depicts the front panel of the LabVIEW control screen used inthe prototype implementation. Here, the user enters the number of rows1205 and columns 1210, and the spacing 1215 between each craniotomy.Also, the user enters where 1220 the data file will be stored.

When the program is started, it takes a few seconds for the motors toinitialize. After they are initialized, they can be moved into positionby using the buttons on the motor drivers or by entering a number forthe displacement of each motor in the LabVIEW window.

The screen also includes interfaces 1225, 1230, 1235 for theservomotors. When the servomotors are initiated, the fields are filledin with numbers. After initialization, numbers can be entered to specifythe location of the motors in fields 1240, 1245, 1250.

Once the drill is positioned in the location for the first craniotomy,the user presses the “Done Positioning” button 1255 and the automatedcraniotomy robot begins drilling the first hole. It will proceed todrill all the holes, and when it finishes, it will retract the drill tothe maximum height and center the other two axes to prepare for the nextprocedure.

FIGS. 13-20 are schematic flowcharts and diagrams relating to theoperation of the example implementation of the custom-built systemaccording to the invention. The LabVIEW visual interface is based on aflat sequence structure, so that it operates in a linear manner whereeach frame of the program executes one after the other, from left toright.

FIG. 13 is a schematic flowchart depicting the steps by which the systeminitializes the motors and then waits for the user to position the drillin the correct location. The blocks labeled “MG17MOTOR” are used tointerface with the motors. First, the motors are sent a serial numberthat corresponds to each different motor. Next, they receive a startcommand. The final motor block in this frame is the “GetPosition” block.This gets the position of each motor and stores it in two localvariables: position and origin. Since these motor blocks are in a whileloop, as the user moves the motors the position is constantly beingupdated. Only after the “Done Positioning” button is pushed, do theposition values get stored in the two previously mentioned localvariables. Another item to note is the reference out local variables.These are used throughout the program whenever a “MG17MOTOR” block isused. These store information that the motor controllers need so it isimportant that they are passed between the different motor blocks.

FIG. 14A is a schematic flowchart depicting the steps by which thesystem moves the drill into position and prepares to perform thedrilling. There are several nested loops that move the drill to eachposition and automatically drill down through the skull. The outermostloop is a loop that does the columns, or x-coordinates for thecraniotomies. As seen in FIG. 14A, it moves the x motor the distance“delta x,” unless it is the first column. If it is the first column, theconditional statement tells the motor to move a step size of zero sincethe drill is already in the desired location for the first column ofcraniotomies. The “MoveJog” command is used for the first time here.This gets an input of either 1 or 2 for the direction (in this case a2).

After the drill is moved to the column location, it is next moved to therow location, as seen in FIG. 14B. As before, if it is the first row,the motor does not move, since the drill is already in the correctlocation. Otherwise, the drill moves over the distance “delta y.”Additionally, instead of moving in the same direction each time, the ymotor changes direction for each column.

For efficiency, in a preferred embodiment, the drill moves in thepattern depicted in FIG. 15. This saves a small amount of drilling timebecause the skull curvature in the x-direction is greater than that inthe y-direction. A greater curvature in the x-direction means that thedrill must be retracted a large distance in order to safely move in thex-direction without hitting the skull. Retracting the drill more meansthat the drill may have to take more steps before creating a craniotomy,which leads to a longer drilling time. By moving the robot as in FIG.15, a smaller z offset is used between the craniotomies in the ydirection. This is just a minor optional feature, but does save sometime, especially for large numbers of craniotomies.

FIG. 16 is a schematic flowchart of another optional procedure used totry to save some drilling time. After a craniotomy is made, the depth atwhich the drill stopped is recorded and the next craniotomy is drilledto this depth plus an offset of either 300 or 400 μm. The skull is onlyabout 150 μm thick, but due to the curvature of the skull, a 300-400 μmoffset is required to ensure that the drill does not go too far on thisstep. For this step, a digital on signal is sent to activate the drill,then the z motor moves the drill down, and finally the drill is turnedoff. There are pauses in each of the frames in this sequence to ensurethat each step is finished before the next one starts.

FIGS. 17A and 17B are schematic flowcharts depicting the steps by whichthe system actually performs the automated drilling. As shown in FIG.17A, the first thing that happens is a measurement of the voltageamplitude is made. This is done using a discrete Fourier transform,which allows the detection signal to be a very low current signal byextracting the information from a noisy signal. The “for” loop in FIG.17A is executing the discrete Fourier transform. The voltage amplitudeobtained from this is compared to the threshold value, and this is usedto make a decision in the next step. As shown in FIG. 17B, if themeasured amplitude is greater than or equal to the threshold, this meansthat the increase in conductance associated with breaking through theskull has not occurred, and the robot must drill further. This sequenceis similar to the one previously mentioned. The drill is turned on, thez motor moves down 5 μm, and then the drill is turned off.

FIGS. 18A-B are schematic flowcharts depicting the steps by which thesystem determines whether or not drilling should be performed. As seenin FIG. 18A, if the measured voltage amplitude is below the threshold,the drill is not turned on, and the z motor retracts to a safe distanceabove the skull to move the drill to the next position. This stops the“while” loop that contains the amplitude measurement and drillingsequence, and allows the program to move to the next drilling position.As seen in FIG. 18B, each time a voltage amplitude measurement is made,it is recorded along with the depth at which it was made, includingsaving the depth location of the finished craniotomy when the voltageamplitude is below the threshold. This is stored in a .txt file.

FIGS. 19 and 20 are schematic flowcharts depicting the steps by whichthe system retracts the drill and completes the program. After all thecraniotomies have been made, the drill is retracted in the z-directionup to the maximum distance (FIG. 19). Then, the x and y motors arecentered (FIG. 20). Finally, all three motors are stopped. Thiscompletes the automated craniotomy program.

Modified Commercial System Implementation (FIGS. 21-24)

To maximize the utility of the design, whether a commercially availablecraniotomy robot that normally operates in open-loop mode could also bemodified to utilize this closed-loop method was explored. In oneembodiment, the invention was implemented using a modified version of acommercially available motorized stereotaxic apparatus, a NeuroStar(Tubingen, Germany) stereotaxic robot. The NeuroStar was modified andfound to be capable of creating reliable, clean craniotomies using thesame conductance-based feedback method.

FIG. 21 is a schematic of an example implementation of a system forperforming automated craniotomies utilizing a modification of theNeuroStar motorized stereotaxic, according to one exemplaryimplementation of the invention. As shown in FIG. 21, a graphical userinterface (GUI) 2105 controls the movement of stereotaxic frame 2110 via3-axis control box 2115. Micromotor carving drill 2120 with adjustablerotation speeds up to 45,000 RPM is attached to stereotaxic 2110 viacustom adapter 2125. Drill 2120 turns end mill 2130 with a tip diameterof 200 μm. When the end mill breaks through the skull, it completes thecircuit formed between a wire carrying the 100 Hz test signal from dataacquisition board 2135 and test lead 2140 connected to the animal 2145.Signal wire 2150 is attached to drill bit 2130 via ball bearing 2155,allowing continuous impedance testing without the need to stop thedrill.

The NeuroStar contains three separate stepper motors, one for each axisof movement in a standard sterotaxic frame. These motors are attached toa Kopf Model 900 Small Animal Stereotaxic Instrument (David KopfInstruments, Tujunga, Calif.). Each motor is connected to a controlunit, which is in turn connected via a USB interface to a computer.

The stepper motors operate by rotating the shaft of the Kopf stereotaxicpositioner to drive movement along that axis, much in the same way anoperator would use the same manual stereotaxic frame. The standarddeviation of the specified steps was measured and found to be 1.8 μm forthe rostral-caudal (X) axis, 2.0 μm for the medial-lateral (Y) axis, and1.2 μm for the dorsal-ventral (Z) axis. The range of travel along eachaxis is 70 mm, which is slightly more limited than that of the Kopfframe, to protect the motors from damage.

As purchased, the NeuroStar robot was only capable of operating in“open-loop” mode, so it was modified to respond to changes inconductance at the drill tip. In one implementation, the NeuroStar drillwas replaced with an electrical micromotor carving tool (Ram Products,Inc., East Brunswick, N.J.) capable of achieving higher rotation speeds(up to 45,000 RPM) and accommodating square end mills with shankdiameters of ⅛″, as well as 1/16″ drill bits with adapters. Thisalleviated the need to modify the end mills, but did require a customadapter for attaching the drill to the stereotaxic frame. Unlike thedefault NeuroStar drill, this drill must be controlled manually (i.e.,it cannot be stopped and started by the existing software).

Electric potential measurements were taken with a National Instrumentsdata acquisition board at a rate of 2 Hz, with each measurement lasting100 ms. The drill was moved down in 5 μm increments after every othermeasurement. If power at 100 Hz (based on the fast Fourier transform ofthe incoming signal) exceeded a threshold of 0.01 V², the drill wasretracted to its starting position. Sending the test signal through aball bearing made it possible to take measurements without stopping thedrill, speeding up the drilling process substantially.

The drill bit (a 200 μm end mill, Harvey Tool, Rowley, Mass.) wasconnected to a measurement circuit via a ball bearing (McMaster CarrPart #60355K501) attached with conductive epoxy (MG Chemicals 8331) andfilled with conductive carbon grease (MG Chemicals 846). A second leadwas connected to the animal by placing a wire beneath the skin, orclipping it to a steel head plate. A data acquisition (DAQ) board(National Instruments USB-6001) sent the 100 Hz test signal through thedrill bit while simultaneously measuring voltage on the test lead with a10 kHz sampling rate. The NeuroStar and the data acquisition board wereboth controlled by a custom Python GUI, written using the PyQt4,PyDAQmx, and PyUSB libraries.

A bearing was attached to the bit as previously described. This was donefor two reasons: so that the largest potential drop across the mousecould be detected, and because it was observed that the drill bodyimpedance changed over time. The impedance of the drill was measured asapproximately 50 KΩ when new, and 0.6-1.7 MΩ after some wear. This islikely due to wear of the bearings that leads to an increase in contactresistance to the housing (i.e., race) of the bearings.

In order to implement closed-loop craniotomies by modifying a commercialsystem, it was necessary to replace the drill (optional, butrecommended), build an impedance-measurement circuit, and install thePython control GUI. The software was implemented on a Windows computerwith Python installed.

FIGS. 22A-B are side and top views of the drill bit, depicting steps forcreating an example implementation of the impedance measurement devicefor use in this version of the invention. First, a wire is connected tothe drill bit via a ball bearing. The ball bearing 2210 is slid over theend mill shaft 2220, using conductive epoxy 2225 to connect the innerrace of the bearing to the end mil 2230, and a piece of hookup wire 2235is soldered to the outer face of bearing 2210 (all-purpose flux beingnecessary to solder stainless steel). The inside 2240 of bearing 2210 isfilled with conductive grease, in order to create electrical continuitybetween end mill 2230 and hookup wire 2235. This modification of thedrill bit permits measurements to be taken even when the bit isspinning.

The measurement circuit is constructed using the NI USB-6001 andstandard hookup wire. It can either be made with a prototypingbreadboard or by soldering wires and a 10 MΩ resistor in the appropriateconfiguration. FIG. 23 is an example implementation of a measurementcircuit useable in this implementation. Shown in FIG. 23 are 10 MΩresistor 2310, connection wire 2320 to the ball bearing, and connectionwire 2330 to the animal.

Table 2 is an example parts list for a specific embodiment of a modifiedcommercial implementation of the present invention.

TABLE 2 Description Vendor Model # stereotaxic drill robot Neuro Stardrill robot electric carving tool Ram Products 4161000 200 μm end millHarvey Tool 13908 ball bearing McMaster 60355K501 conductive epoxy MGChemicals 8331 conductive grease MG Chemicals 846 DAQ NationalInstruments NI USB 6001 drill holder custom 10 MΩ resistor various

An example embodiment of a graphical user interface (GUI) that makes itpossible to control the NeuroStar drill from any computer was written inPython and is shown in FIG. 24. The code itself, which represents anexample embodiment only, is contained in the computer program listingappendix that has been submitted in computer-readable format as anelectronically-filed text file under the provisions of 37 CFR 1.96 andis herein incorporated by reference in its entirety.

Installation. The prototype GUI currently works on Microsoft Windows,due to its dependence on NI-DAQmx for controlling the NationalInstruments board, but it is clear that implementations on other systemsand in other languages are within the skill of the artisan and withinthe scope of the invention. The standard libraries (e.g. matplotlib,numpy, PyQt4) are utilized. Because the impedance-feedback circuit is sosimple, in some embodiments custom hardware may be created for thispurpose.

The GUI provides several functionalities through a series of buttons anddrop-down menus. These may include, but are not limited to, thefollowing:

Manual control. The left-hand side of the GUI holds buttons for manuallymoving the 3 axes of the NeuroStar. Each axis has “long” (two arrows)and “short” (one arrow) movement controllers. By default, long steps=1mm, short steps=20 microns.

“1× distance”—toggles the setting for movement distance; in 0.5×distance mode, long steps=0.5 mm, short steps=10 microns

“X”=anterior/posterior control, “Y”=medial/lateral control,“Z”=dorsal/ventral control

“Bookmark” button—saves coordinates for returning to later; will promptthe user for a string to label that particular coordinate

“Re-center” button—zeros out the axes, so that future coordinates aredisplayed relative to the current coordinate

“Stop” button—prevents the NeuroStar from executing the next movement(it cannot stop the current movement)

The drill may be moved a precise distance along any axis by clicking the“GO TO . . . ” button.

Drilling with feedback. The center of the GUI holds the interface forimpedance-based feedback. The left plot shows 100 ms of raw data fromthe test lead connected to the animal. Before the drill has penetratedthrough the skull, this should show a 100 Hz sine wave, perhaps withsome line noise contamination. After penetration, the amplitude of thissignal should decrease dramatically, signifying a change in impedance atthe drill tip. The right plot shows the frequency-domain representationof the signal. When the power at 100 Hz dips below a threshold(indicated by the bottom of the yellow box overlay), the drill willretract to its original location.

“Drill!”—starts the process of lowering the drill while checking for a100 Hz threshold crossing. When the threshold is reached, the drill willreturn to where it was when the button was pressed. Pressing the buttona second time will interrupt the process.

“Interval (ms)”—determines how long the software will pause betweenchecking for threshold crossings. The NeuroStar will only move down in Zafter every other check (so the movement interval=2× the indicatedinterval).

“Step size (um)”—determines how far the NeuroStar will move during eachmovement step. 5 um is the recommended distance when actually drillingthrough the skull (to prevent unnecessary damage to the underlying braintissue), but larger increments can be used to position the drill justabove the skull surface.

“Amplitude”—determines the amplitude of the sine wave (in volts)

“Threshold”—power at 100 Hz below which indicates the drill has brokenthrough the skull.

The drilling parameters (interval, step size, amplitude, and threshold)can all be adjusted online.

Pattern interface. The left of the GUI holds buttons for programmingmore complex drill patterns, such as grids and circles.

“Start!”—initiates the process of drilling holes in the patterncurrently indicated by the interface

“Pattern”—toggles between “circle” and “grid”

“Width (um)”—determines the width of the entire pattern in microns

“Number of points”—the total number of points in the pattern (when in‘grid’ mode, there are options for both rows and columns)

As the pattern is being drilled, the software records the Z height ofthe brain at each point, allowing the user to then interpolate smoothlybetween all of the points along the path. Interpolation only works wellin “circle” mode, and is not meant to be used in “grid” mode.

“Interpolate”—initiates the process of drilling a smooth path betweenthe holes, using the parameters currently indicated by the interface

“Interp height”—the height of the interpolated path in microns relativeto the measured brain surface (negative=above the surface,positive=below the surface, consistent with the conventions of the Zaxis moving up for negative values). The recommended value for avoidingdamage while making the skull easy to remove is −20.

“Interp points”—number of points in the interpolation path. Largernumbers=smoother but slower.

“Start” and “end”—by default the interpolation goes around the entirecircle (from point 1 back to point 1), but segments may also be manuallyselected to be drilled at a different depth. This can also be used tore-drill specific holes in the pattern (and thereby re-calculate the Zheight at that point).

“Drilling” versus “milling”—toggles between two interpolation patterns,one in which the drill moves vertically through the skull (drilling) andone in which it moves horizontally (milling). Drilling verticallythrough the bone is highly recommended, as it tends to produce cleanercraniotomies.

Experimental Results (FIGS. 25A-B-28A-B)

As previously discussed, the invention leverages the principle that adrill bit passing through the mouse skull encounters a stereotypedincrease in conductance with respect to the mouse body. A method basedon this information is utilized within the invention to detect skullbreakthrough. Both the custom and modified commercially availablerobotic devices, described earlier, that utilize this method performedautomated craniotomies with high precision, high yield, and good safety,with the ability to stop with ˜5 micron resolution.

Brain bleeding was not observed in any of the trials performed by therobot once the appropriate conductance threshold was determined. Evenminor bleeding can compromise vasculature important for corticalmaintenance [Shih A. Y., Blinder P., Tsai P. S., Friedman B, Stanley G,Lyden P D, Kleinfeld D. The smallest stroke: occlusion of onepenetrating vessel leads to infarction and a cognitive deficit. NatNeurosci. 16(1): 55-63, 2013], and release of blood can beneuromodulatory or even toxic to neurons [Yip S., Ip J. K., Sastry B. R.Electrophysiological actions of hemoglobin on rat hippocampal CA1pyramidal neurons. Brain Res. 713(1-2): 134-142, 1996; Regan R. F., GuoY. Toxic effect of hemoglobin on spinal cord neurons in culture. JNeurotrauma. 15(8): 645-653, 1998]. Precise control of drill depthobviates such concerns.

FIGS. 25A and 25B depict an example one of the large windows that werecreated after drilling a series of test holes to account for thecurvature of the skull. In this example, holes were drilled at sevenpoints along the circumference of a 3 mm circle, forming the desiredwindow. Impedance-based feedback was used to measure the location of thebrain along the z-axis at each point. Then, cubic spline-basedinterpolation was used to compute the optimal path for the drill to milla circular pattern in the bone without contacting the underlying tissue.

When the drill broke through the skull (as indicated by a change inconductance), the depth was recorded to obtain a Z depth for each X-Yhole location. Once all of the test holes were drilled, the softwarecomputed a trajectory based on cubic spline interpolation of the X, Y,and Z coordinates of each hole (500 points per axis). The drill thenfollowed this trajectory in “open-loop” mode to cut a circular hole inthe skull while avoiding damage to the underlying tissue by milling adistance of 20 microns above the measured Z depths.

FIG. 26A-D depict the steps involved in creating two cranial windows inan actual mouse skull. In FIG. 26A, the skull is exposed and cleaned. InFIG. 26B, the center point and diameter of the desired cranial window ismanually chosen by the surgeon, and seven holes are automaticallydrilled along its circumference. In FIG. 26C, the drill automaticallyinterpolates between the hole locations at the appropriate depth, andthe skull is manually removed under saline. In FIG. 26D, the steps ofFIG. 26A-C are repeated for the opposite hemisphere

The system was used to create larger cranial windows up to 5 mm indiameter (N=5 3 mm windows in 4 mice, N=2 5 mm windows in 2 mice). Usinga 200 μm square end mill, a series of test holes were drilled around thecircumference of the desired window (FIG. 26B). Based on the measured X,Y, and Z coordinates of each hole, the drill was swept along a path thataccounted for the curvature of the skull. The bone fragment was thenremoved under saline, exposing the brain underneath (FIG. 26C). Theprocedure was repeated for the opposite hemisphere without complications(FIG. 26D). The entire process took approximately 15 minutes for eachwindow (10 minutes to drill the test holes at 1 minute and 20 secondsper hole, and 5 minutes to open the window).

All procedures were in accordance with the National Institutes of HealthGuide for the care and use of Laboratory Animals and approved by theMassachusetts Institute of Technology and the Allen Institute for BrainScience Animal Care and Use Committees. Two- to five-month-old C57BL/6wild-type mice (male) were given general anesthesia using a rodentanesthesia machine with 2% isoflurane in pure oxygen. Animals wereplaced on a heating pad with a temperature probe to maintain bodytemperature. After fully non-responsive to foot withdrawal reflex test,mice were administered the analgesics buprenorphine (0.1 mg/kg) andMeloxicam (1-2 mg/kg) subcutaneously. Mice were immobilized in astereotaxic apparatus with ear bars, and a nose holder and bite bar.Using a scalpel, an incision was made on the scalp to expose the skull,retracting the skin using clips. A small curette was used to retract orremove residual fascia or connective tissue over the area of interest ofthe skull. Experiments lasted up to 2 hours, with saline only added tosoft tissues so that the skull would present a constant dry impedance.Experiments were terminal. After the end of each experiment performedwith the custom-built system, skulls were removed, and craniotomydiameters were measured using an x-ray micro-computed tomography system(XT H 160, Nikon, Tokyo, Japan).

When the drill tip came in contact with the skull surface, the observedelectrical current was negligible, due to the small conductance of theskull (<0.10 nS; measured with an LCR meter, 4263B, Agilent, SantaClara, Calif.). However, when the drill tip penetrated the skull, theconductance between the drill bit and the body dramatically increaseddue to the high conductivity of cerebrospinal fluid. To quantify thiseffect as a function of frequency, the electrical impedance was measuredbetween a dental drill and the body of a mouse as the drill created acraniotomy, using a custom circuit (FIG. 2) that performed thismeasurement. A simple robot was constructed to lower the drill bit (FIG.5) in a controlled fashion (FIGS. 3, 4). A sinusoidal test signal wasdelivered to the drill, and measured the voltage amplitude and phaseangle across the mouse as a 500 μm diameter dental burr was passedthrough the skull, for eight different signal frequencies from30-100,000 Hz (2, 5, 6, 10, 15, 18, and 19 sweeps per mouse). It wasfound that the voltage was sensitive to drill depth through the skullover a broad range of frequencies from 30 Hz to 1 kHz (n=7 mice; FIG.6A, voltage data; FIG. 6B, calculated conductance data). Based on thesedata, a frequency of 100 Hz was chosen because it showed the largestelectric potential drop as the drill bit passed through the skull.

Using a 100 Hz sinusoidal voltage, the time course of the conductancechanges as the drill advanced through the skull was examined. FIG. 27Ais a graph of normalized electric potential vs. distance traveled, for10 holes in one mouse skull, each represented by a different color (stepsize 5 μm, frequency 100 Hz). Traces were aligned (at distance=0) at thepoint in the curve of maximum slope. FIG. 27B is a graph of electricalconductance vs. distance traveled (as in FIG. 27A) for all 10craniotomies (n=10 holes in one mouse skull).

Using the robot of FIGS. 2-4, it was observed that (for each craniotomy)conductance is near zero for most of the drilling process, then jumpssignificantly to a higher conductance over a small number of steps, andthen remains high for subsequent steps of the drill. This indicates thatconductance does not vary appreciably as the skull is thinned, butinstead rises suddenly when the drill passes slightly through the innersurface of the skull. It was necessary to determine whether a precisethreshold for the conductance could be derived, so that the robot wouldstop when it completed the craniotomy without damaging the brain.

FIG. 28A is a plot of hole size, measured at the base of the skull,measured using x-ray micro-computed tomography (CT), as a function offinal normalized electric potential, with the drill stopping whenvarious normalized electrical potentials were reached. n=98 craniotomiesin 5 mice; 200 μm drill bit (width indicated by dotted line). Each mouseis represented by a different shape, with red fill indicating visibleblood arising near the stopping point. For 6 of the 98 craniotomies, thedrill bit did not pass the bottom of the skull, and thus are on the y=0line. FIG. 28B is a plot of hole size vs. electrical conductance, forthe data of FIG. 28A.

The robot was operated with a 200 μm drill bit, stopping the drillingwhen various drill-to-mouse electrical potentials (FIG. 28A, normalized;FIG. 28B, calculated conductances) were achieved. For each trial, thecraniotomy diameter at the base of the skull was measured. As thenormalized electrical potential threshold was lowered systematically(from ˜0.95 down to ˜0.3), the hole diameters increased, from under 150mm to around 200 mm (n=98 craniotomies in 5 mice). The electricpotential and hole diameter were inversely correlated (FIG. 28A,correlation coefficient r=−0.61, p=1.26×10⁻⁷) with lower potential beingassociated with larger hole diameters. In 6 of the 98 cases, theexperiment was stopped because of hitting a blood vessel in the skull—abenign, but unavoidable, occasional event that yielded zero-diameterholes. In a small minority of cases, the drill successfully created acraniotomy but produced bleeding, suggesting a sub-optimal craniotomy,perhaps related to damage caused by the use of a standard pointed drillbit (red symbols, FIGS. 28A-B). A threshold for the drill was chosenthat was high enough so that bleeding could be avoided, but small enoughto maximize hole size. For the particular 200 μm diameter drill bitsthat were used in these experiments, it was found that a normalizedelectric potential threshold value of 0.65 yielded a good balance.

In the experiments of FIGS. 27A-B and 28A-B, the circuit works bysending a sine wave from a function generator (20 V, 100 Hz) through asense resistor (681 kW) and a wire in contact with the drill body. Thecable conducts the electrical signal from the function generator to thedrill and from the mouse back to the DAQ. This cable consisted of a pairof twisted wires. The rear paw of the mouse made electrical contact tothe ground lead of this cable connected to the ground pin of the DAQthrough a piece of metal touching the skin of the paw, and heldstationary by a test clip whose spring had been stretched to make itweaker.

The DAQ measures V_(n) as follows: the Lab VIEW program records themaximum and minimum value of ten sinusoidal waves, each consisting of1,500 samples at 25 kHz. It subtracts the minimum value from the maximumand averages these ten to estimate the peak-to-peak amplitude of thevoltage drop across the mouse. These two methods only work when thesignal amplitude is much higher than the noise, which is the case whenthere is a large driving current. The impedance of the drill wasincluded in the circuit because the simplest method for sending a signalthrough the drill is to connect a wire to the body of the drill, whichis conductive to the drill bit. This impedance would show up as aresistor in series with the mouse in FIG. 2. Electric potentialmeasurements were made in discrete steps. The solenoid valve was turnedon for 300 μs, a step of 5 μm was taken, a pause of 3.5 s allowed thedrill to come to a complete stop, and then a measurement was taken for0.6 s.

The use of robotic craniotomy robots with this skull breakthroughdetection method allows many small precisely-spaced craniotomies to bedrilled, which may be useful for deploying multi-injector [Chan, S. Y.,Bernstein, J. G., Boyden, E. S. Scalable Fluidic Injector Arrays forViral Targeting of Intact 3-D Brain Circuits. J Vis Exp. 35: 1489,2010], multi-electrode [Recce, M., and J. O'Keefe. The tetrode: a newtechnique for multi-unit extracellular recording. Soc Neurosci Abstr.15(2), 1989; Maynard, Edwin M., Craig T. Nordhausen, and Richard A.Normann. The Utah intracortical electrode array: a recording structurefor potential brain-computer interfaces. Electroencephalogr ClinNeurophysiol 102 (3): 228-239, 1997], or multi-optical fiber probes[Zorzos, A. N., Scholvin, J., Boyden, E. S., & Fonstad, C. G.Three-dimensional multiwaveguide probe array for light delivery todistributed brain circuits. Opt Lett. 37(23): 4841-4843, 2012] forinterrogating distributed neural circuits. The smallest size ofcraniotomy possible with the technique presented here is limited by thesize of available bits, which currently go down to 50 μm, but it may bepossible to create custom tools that are even smaller. It is extremelydifficult for human surgeons to handle drill bits of this size, making arobotic device a practical solution when holes of this size are desired.

Previous techniques are either more expensive or less precise than thismethod. Furthermore, this technique can be easily adapted to an existingsurgery setup that a lab already has. This method for automated openingof craniotomies will be accessible to a wide range of neuroscientists.The method works equally well on custom-built systems and modifiedcommercially available drilling systems. Because the technique isconceptually simple, it should be straightforward to adapt it toalternate setups.

Automated craniotomies should not be thought of as a replacement forhuman surgeons, but rather as a useful addition to the surgical toolkit.The method still requires a human to place the mouse in a stereotaxicdevice, expose the skull, and align the drill with the appropriatestructures. Using the technique for automated craniotomies should resultin more consistent holes with smaller diameters and tighter spacing thanpreviously possible. Furthermore, the opening of large cranialwindows-something that typically requires extensive training-can now beperformed automatically. And if multiple surgical robots are set up inthe same laboratory, the same number of experimenters can perform moresurgeries in less time. As the tools for neural recording andstimulation become increasingly sophisticated, it is important toeliminate variability in craniotomy quality as a potential failure modefor these devices. The use of conductance-based feedback is an effectiveway to improve the reliability of the holes needed to expose the brainprior to inserting pipettes, electrodes, and fiber optic cables, or forthe purpose of imaging neural tissue.

While preferred embodiments of the invention are disclosed herein, manyother implementations will occur to one of ordinary skill in the art andare all within the scope of the invention. Each of the variousembodiments described above may be combined with other describedembodiments in order to provide multiple features. Furthermore, whilethe foregoing describes a number of separate embodiments of theapparatus and method of the present invention, what has been describedherein is merely illustrative of the application of the principles ofthe present invention. Other arrangements, methods, modifications, andsubstitutions by one of ordinary skill in the art are therefore alsoconsidered to be within the scope of the present invention.

What is claimed is:
 1. An automated craniotomy opening apparatus,comprising: a craniotomy drilling apparatus having a drilling tip; atleast one craniotomy drilling apparatus positioning device connected tothe craniotomy drilling apparatus; a detection device connected to thedrilling tip; and a computer processor specially configured andconnected to automatically and iteratively control the craniotomydrilling apparatus, the craniotomy drilling apparatus positioningdevice, and the detection device, the computer processor programmed toautomatically open a craniotomy, without user intervention, by:positioning the drilling tip with respect to the target skull at apredetermined position for drilling; drilling into the target skull withthe drilling tip for a predetermined distance; after drilling,automatically determining the conductance near the drilling tip usingthe detection device; if the conductance is below a predeterminedthreshold, automatically returning the drilling tip to a home position;and if the conductance is not below the predetermined threshold,automatically repeating drilling for the predetermined distance anddetermining the conductance until the conductance exceeds thepredetermined threshold.
 2. The apparatus of claim 1, wherein thedetection device includes an impedance detection circuit.
 3. Theapparatus of claim 2, wherein the impedance detection circuit comprises:a signal source; a sense resistor in series with the signal source; andan output for sending the detected impedance to the computer processor.4. The apparatus of claim 1, wherein the detection device is attached tothe drilling tip in a manner that permits measurements to be made whilethe drilling apparatus is running.
 5. The apparatus of claim 1, whereinthe drilling tip is a blunt-tipped end mill.
 6. The apparatus of claim1, wherein the craniotomy drilling apparatus positioning apparatus is amotorized stereotaxic stage connected to the drilling apparatus.
 7. Theapparatus of claim 2, wherein determining the conductance comprises:under the control of the computer processor, measuring impedance withthe impedance detection circuit; and calculating the conductance usingthe measured impedance.
 8. The apparatus of claim 7, wherein measuringimpedance comprises: under the control of the computer processor,sending a signal through the impedance detection circuit to the drillingtip; detecting a voltage at the target skull; and sending a signalrepresenting the detected voltage from the impedance detection circuitto the computer processor.
 9. The apparatus of claim 8, whereincalculating the conductance comprises determining a voltage drop acrossthe impedance detection circuit.
 10. The apparatus of claim 7, whereinmeasuring impedance comprises the steps of: under the control of acomputer processor, sending a signal at a predetermined voltage throughthe impedance detection circuit to the target skull; detecting a voltageat the drilling tip; and sending a signal representing the detectedvoltage from the impedance detection circuit to the computer processor.11. The apparatus of claim 10, wherein calculating comprises determininga voltage drop across the impedance detection circuit.
 12. The apparatusof claim 1, wherein a drill hole pattern comprising a plurality ofcraniotomies to be drilled in the target skull is predetermined and thedrill hole pattern is created by drilling the plurality of craniotomiesusing the apparatus of claim
 1. 13. The apparatus of claim 12, whereinthe predetermined drill hole pattern is selected to facilitate creationof a cranial window in the target skull.
 14. The apparatus of claim 13,wherein the cranial window is created by drilling along a path thatinterpolates between the holes to form a circumference of the cranialwindow.